Perinatally HIV-infected children provide a unique model system to study the chronic effects of growing up with HIV. We propose to re-enroll 500 HIV-infected children age 5-9 years at two treatment sites in Johannesburg, South Africa. These children are prior participants in clinical studies who initiated therapy during the first two year of life and have detailed clinical information, HIV-related laboratory results and over 10,000 blood specimens stored in repositories. We will prospectively follow the re-enrolled children and collect standardized clinical data on co-morbidities, toxicities and complications, laboratory data on HIV-related parameters and blood samples and other specimens for special studies. We will also enroll a cross-sectional sample of 250 uninfected children (household/sibling controls) frequency-matched by age within 3-month intervals to the re- enrolled children at the time of recruitment. We will integrate the newly-collected, follow-up data and samples with the historical databases and repositories to provide a platform to investigate (1) host epigenetics and (2) mitochondrial function and their relations with HIV disease progression, drug regimens, and metabolic complications in HIV-infected children. First, using well-established array-based methods and pyrosequencing, we propose to identify the spectrum of gene-specific DNA methylation changes that accompany perinatally- acquired HIV infection and HIV treatment. Second, utilizing both stored and newly-collected specimens, we propose to test whether mitochondrial dysfunction (as measured by novel methods of enzyme function and cell stress) exacerbates chronic inflammation and immune senescence underlying HIV disease progression and the metabolic complications that may accompany long-term treatment. Our prospective cohort, building on an extensive and well-characterized perinatally-infected population, will provide a valuable resource to undertake these and future pathogenesis-oriented studies to inform development of complementary interventions to improve long-term outcomes of HIV-infected children in sub-Saharan Africa.